1. Field of the invention
The invention relates to a method for thickness measurement as well as to an apparatus for implementing such a method and more generally to a method of linearization of an exponential function as well as to an apparatus for the implementation thereof.
An apparatus for measuring the thickness of an object usually comprises a radiation source having a known intensity disposed on one side of the object and a radiation detector on the other side thereof. The frequency of the pulses delivered by the radiation detector is inversely related to the intensity which has been attenuated as the result of the absorption by the object the thickness of which is to be measured.
2. Description of the prior art
Apparatus of this type are known. As a source of radiation, a gamma radiation or a radioactive isotope can be used, for example, isotope Cs.sup.137 is used if thick sheets of metal are to be measured. As radiation detectors, it is well known to use ionization chambers or scintillators coupled to a photomultiplier. The latter is usually preferred because it permits operation with a narrow collimated beam which alleviates some problems of protection against radiation. However, in both cases, the output of the radiation detector is a sequence of statistically distributed pulses.
In prior art apparatus this pulse sequence is converted by a transducer into an analog singal the magnitude of which is proportional to the average number pulses within a time interval. In order to operate at the same point on the characteristic curve whatever the thickness of the object may be, absorption sheets having known absorption are introduced across the beam, and the number of absorption sheets which are so introduced is taken into consideration for determining the actual thickness of the object to be measured.
The main drawbacks of this type of apparatus are, on the one hand, the use of a mechanical device for introducing the absorption sheets, which is a source of trouble in a rolling mill with its inherent rough environmental conditions and, on the other hand, the measuring errors due to drift and offset of the analog circuits.
It is the primary object of the invention to provide a method of and an apparatus for measuring the thickness of an object in which the above-mentioned drawbacks are eliminated.